Show an error on startup if a bitcoin datadir that is being used contains a
`bitcoin.conf` file that is ignored. There are two cases where this could
happen:

- One case reported in
  bitcoin#27246 (comment)
  happens when a bitcoin.conf file in the default datadir (e.g.
  $HOME/.bitcoin/bitcoin.conf) has a "datadir=/path" line that sets different
  datadir containing a second bitcoin.conf file. Currently the second
  bitcoin.conf file is ignored with no warning.

- Another way this could happen is if a -conf= command line argument points
  to a configuration file with a "datadir=/path" line and that specified path
  contains a bitcoin.conf file, which is currently ignored.

This change only adds an error message and doesn't change anything about way
settings are applied. It also doesn't trigger errors if there are redundant
-datadir or -conf settings pointing at the same configuration file, only if
they are pointing at different files and one file is being ignored.

Currently debug.log will show the wrong bitcoin.conf config file path when
bitcoind is invoked without -conf or -datadir arguments, and there's a default
bitcoin.conf file which specifies another datadir= location. When this happens,
the debug.log will include an incorrect "Config file:" line referring to a
bitcoin.conf file in the other datadir, instead of the referring to the actual
configuration file in the default datadir which was parsed.

The bad log print was reported and originally fixed in
bitcoin#27303 by
Matthew Zipkin <pinheadmz@gmail.com>

This PR takes a slightly different approach to fixing the bug, trying to avoid
future bugs by not allowing the GetConfigFilePath function to be called before
the the configuration is parsed, and deleting GetConfigFile function which
could be confused with GetConfigFilePath. It also includes a test for the bug
which the original fix did not have.

Co-authored-by: Matthew Zipkin <pinheadmz@gmail.com>

Currently bitcoin-wallet accepts a -datadir argument but ignores and config
file in that directory. This changes it to load the config file for consistency
with other cli tools, and in case it contains any significant settings.

Also update various tests using the data directory to call ReadConfigFiles.

Motivation for these changes is to prevent errors after the next commit makes
it at assert error to get the datadir path without reading the configuration
first.

It's always been possible for bitcoin default datadirs ($HOME/.bitcoin,
$HOME/Library/Application Support/Bitcoin, %APPDATA%\Bitcoin) to point at an
external locations using symlinks, but not all filesystems support symlinks, so
add extra support for pointing at external locations using text file.

This feature is used in the following commit to allow the bitcoin GUI to select
a default datadir location that will also be treated as the default datadir for
CLI tools. Currently when a custom data is set in the GUI, CLI tools ignore it
by default.

Currently if a you choose a non-default datadir in the GUI intro screen, the
datadir is ignored by CLI tools. This means `bitcoin-cli` and `bitcoin-wallet`
will try to use the wrong datadir and show errors if they are called without a
-datadir arguments, and `bitcoind` will appear to work but use a different
datadir, not loading the same wallets and settings, and downloading blocks into
the wrong place.

There are also more subtle inconsistencies between GUI and CLI selection of
datadirs such as bitcoin#27273 where GUI might ignore a datadir= line in a
bitcoin.conf that CLI tools would apply.

This PR gets rid of inconsistencies between GUI and CLI tools and makes them
use the same datadir setting by default. It is followup to
bitcoin-core/gui#602 which made GUI and CLI tools use
the same `-dbcache`, `-par`, `-spendzeroconfchange`, `-signer`, `-upnp`,
`-natpmp`, `-listen`, `-server`, `-prune`, `-proxy`, `-onion`, and `-lang`
settings as long as they loaded the same datadir.

The reason for GUI and CLI tools using inconsistent datadirs, is that GUI
stores the datadir path in a `strDataDir` field in
`.config/Bitcoin/Bitcoin-Qt.conf`[^1] which CLI tools ignore. This PR changes
the GUI to instead store the datadir path at the default datadir location
`~/.bitcoin`[^2] as a symlink that CLI tools will already follow, or as a text
file if the filesystem does not support creating symlinks.

If upgrading from a previous version of the GUI and there is only a GUI
datadir, the `strDataDir` setting will be automatically migrated to a symlink
so CLI tools will start using it as well. If CLI and GUI tools are currently
using different default datadirs, the GUI will show a prompt allowing either of
the datadirs to be loaded and optionally set as the common default going
forward.